I'm planning to stand up a new 5.0 environment (all on VMWare, except the SQL DB) and use Enterprise Transporter to move users. The fact that it doesn't work with 4.3 was going to be a pain point because we have several hundred, but our client support team has decided to upgrade all device in our corporate office (1300+) to the newest firmware; they're plan is to allow some (technically minded) users to upgrade their own via WDM and do the rest themself with 'regular' Desktop Manager, assembly line style.
I haven't planned out any details yet. We currently have 3 BES in one BB domain, hosting 2500+ users, and another BES in a seperate BB domain that I plan to collapse into the first BB domain hosting 250 users. I'm thinking at minimum, we'll have 3 BES (specficailly, 3 pairs of them, HA style) and another with BAS and possibly WDM (currently WDM is running off an old test box).
I'm interested in seeing some details on peoples' implementations once they are up and running.